1. Field of the Invention
This invention relates to a process for the recovery of hydrocarbons from subterranean hydrocarbon-bearing formations containing sandstone and clay components in which the permeability and porosity of the formation communicating between the producing well and adjacent injection well are increased by treating with an aqueous solution of an acid such as hydrochloric acid, etc. and a fluorine-containing acid or salt and having dissolved therein an oxyalkylated acrylamido alkanesulfonic acid polymer thereby facilitating the flow of fluids through the formation resulting in increased hydrocarbon recovery via the production well.
2. Description of the Prior Art
In recovering oil from oil-bearing reservoirs it usually is possible to recover only a minor part of the original oil in place by the primary recovery methods which utilize the natural forces present in the reservoir. As a result, a variety of supplemental recovery techniques have been utilized to increase the recovery of oil from subterranean hydrocarbon-bearing reservoir or formation. Although these supplemental techniques are commonly referred to as secondary recovery operations in fact they may be primary or tertiary in sequence of employment. In such techniques, a fluid is introduced into the formation in order to displace the oil therein to a suitable production system through which the oil may be withdrawn to the surface of the earth. Examples of displacing mediums include gas, aqueous liquids such as fresh water or brine, oil-miscible liquids such as butane or a water and oil-miscible liquid such as an alcohol. Generally, the most promising of the secondary recovery techniques is concerned with the injection into the formation of an aqueous flooding medium either alone or in combination with other fluids.
In application of these conventional procedures for the production of hydrocarbons from similar formations by the secondary recovery method of water-injection, one of the principal difficulties that has been encountered is the generally low production response realized because of the low permeabilities and the consequent low rate of water acceptance of the communicating formation. Thus, these unfavorably low responses both in injection rate and in overall production have led to the abandonment of hydrocarbon production by water-injection methods from many formations containing sandstone and clay components after only a minimal amount of the oil-in-place has been produced.
Treatment of sandstone formations by mixed hydrochloric-hydrofluoric acids has been used in the past as a means of removing damage caused by the presence of clays either originally present in the formation or introduced into the formation during drilling operations. The removal of such clays is accomplished by dissolution by reaction with the hydrofluoric acid: EQU 36HF+Al.sub.2 Si.sub.4 O.sub.10 (OH.sub.2)43 4H.sub.2 SiF.sub.6 +12H.sub.2 O+2H.sub.3 AlF.sub.6.
Furthermore, the hydrofluoric acid component of the mixed acid will react with the sand and other siliceous minerals according to the following equation: EQU 6HF+SiO.sub.2 .fwdarw.H.sub.2 SiF.sub.6 +2H.sub.2 O.
The dissolution of both clays and siliceous material can materially increase the permeability and porosity of the formation and enhance the injectivity of wells utilized in secondary recovery operations. Although treatment of the formation in the vicinity of the injection well bore usually results in an increase in the flow capabilities of the formation, the response in production improvement may be only temporary.
It has been widely assumed that these mixed acid systems could be injected into a sandstone formation to dissolve clay at almost any depth from the wellbore, and that their reaction upon silica surfaces is so slow that little reaction takes place. A recent study (J. Pet. Tech., Vol. XXII, June 1970, p. 693) has shown that the reaction rate of mixed HCl-HF on clays is virtually instantaneous. Since clay is the mineral that usually causes reduction in fluid injection rates into sandstone formation, retarding the reaction rate of the acid mixture toward both sandstones and clays would be greatly beneficial. There are two types of mud damage: (1) Completion damage, which results from drilling mud permeating the pore spaces of the formation adjacent to the wellbore, which is generally limited to a zone extending about one inch from the wellbore, and (2) Natural damage, which results from a reduction in virgin permeability as a result of swelling or migration of sensitive clays which may cause plugging of the formation flow channels. Natural damage is usually thought to exist to a depth of 2 or 3 feet from the wellbore. However, it can extend 7 or 8 feet or more from the wellbore. One can see that when treating formations with natural clay damage, the production increase realized is dependent upon the distance which the hydrofluoric acid can be pumped into the formation before being spent. Furthermore, as the depth of damage increases, the need for an acid mixture which will attack the formation at a greater distance from the wellbore becomes more desirable.
One method suggested to overcome the abovementioned disadvantage of the fast reaction of the acid solution in the vicinity of the wellbore has been the use of "retarded" acids which consist, for example, of mineral acids and an additive which emulsifies the acid, a combination of which affects the acidization rate. Although such emulsified mixtures can be displaced into the formation before substantial reaction occurs, such compositions have the inherent disadvantage in that when the emulsion breaks and they do react, they usually react swiftly, often unpredictably with the result that problems of cavitation and channeling develop. More importantly, the use of such emulsified retarded acid has been limited to conventional acidization processes with hydrochloric acid alone, which has no utility in the dissolution of silica or clays.
The use of cross-linked copolymers of polyvinylpyrrolidone and polyacrylamides, polyurethanes, etc. to give materials which are insoluble in aqueous mineral acid solutions is described in U.S. Pat. No. 3,380,529 to Hendrickson. Such insoluble, cross-linked polymers are utilized as agents for partially plugging channels developed during acidization treatment in order to effect acidic attack at greater distances from the wellbore. In U.S. Pat. No. 3,432,971 a similar acidization process is described in which copolymer prepared by polymerizing acrylamide and N-vinylpyrrolidone in the presence of a cross-linking agent, such as N,N'-methylenebisacrylamide, is disclosed. These copolymers are insoluble in the aqueous mineral acid solutions employed in acidization operations and are utilized as dispersions in the acid solutions. Both of the previously mentioned acidization processes which utilize insoluble, cross-linked copolymers are distinctly different from the novel method of this invention in which a polymer soluble in aqueous mineral acid solutions is used.
The primary object of the present invention is to provide a process for the improved recovery of fluids and especially hydrocarbons from subterranean fluid-bearing formation by providing a process wherein a composition comprising an aqueous solution of a mineral acid, such as hydrochloric acid, and a fluorine-containing acid or salt, and having dissolved therein an oxyalkylated acrylamido alkanesulfonic acid polymer or copolymer, is injected into a formation communicating between a producing well and an adjacent injection well, said formation containing sandstone and clay components and whereafter the acids and acid salts contained in the said composition react with the acid-soluble components of the formation to increase permeability and porosity of the formation thereby facilitating the flow of fluids therethrough.